1. Field of the Invention
The present invention relates generally to electrically conductive polymeric compositions. More particularly, the present invention involves a polymeric matrix material incorporating a filler of intrinsically conductive polymer coated carbon particles useful for fabricating devices capable of long term electrical charge dissipation.
2. Description of Related Art
Organic polymers are generally good insulators and have a high value of specific resistance. Materials having these insulating characteristics, such as plastic devices and polymeric films and powders, accumulate electrostatic charges which can produce sparks and potentially cause explosions and fires in certain environments. A particularly troublesome source of accumulated surface charge occurs as the result of friction created between moving surfaces. The static charge build-up can occur between moving fluids in contact with other materials, either liquids or solids. Thus, for example, when fuels or other volatile chemicals are transported through lines of insulating material, large surface voltages can build-up on the chemical and the insulating material. Under some circumstances, the material or fuel can discharge its energy to ground in a spark capable of igniting any volatile material in the surrounding environment. Additionally, the static potential can be high enough to actually cause a discharge through a chemical transport line, for example, a fuel line, creating a pinhole. Sparking caused by subsequent charging can ignite fuel or volatile material leaking through the pinhole with catastrophic results.
Since it is not practical to prevent the friction which causes the charge generation, remedial measures for decreasing the possibility of fires and explosions are based on increasing the rate of charge dissipation or charge leakage so that charge build-up does not occur. Along these lines, one approach is to utilize inherently electrically conductive materials such as metals to fabricate fuel lines. However, due to weight, flexibility and cost considerations transport lines fabricated from polymeric materials are preferred.
A more commonly utilized approach is to utilize polymers or polymeric compositions incorporating electrically conductive filler materials to fabricate chemical transport lines which are electrically conductive.. The incorporation of conductive filler material into polymer formulations increases the electrical conductivity of the whole polymeric material. The overall function of conductive filler material involves decreasing the rate of charge generation or increasing the rate of charge dissipation, or both mechanisms. Thus, the conductive nature of these polymers and polymeric compositions contributes to the prevention of possible catastrophic explosions and fires during the transport of volatile fuels and chemicals.
Common electrically conductive filler materials utilized to impart electrical conductivity to polymers include carbon based particles. Carbon black in particular is widely utilized by polymer compounders to vary the electrical characteristics of polymers. By selecting suitable carbon black filler and carefully formulating the carbon black with an appropriate polymer, a wide range of conductive polymeric compositions can be fabricated. Typically, finished polymeric products can be made electrically conductive by effectively dispersing carbon filler into the polymer prior to fabricating the finished product. With respect to electrically conductive chemical transport lines, carbon black of varying graphitic structure is incorporated into a suitable polymer, for example nylon. Then, the resulting electrically conductive formulation is generally extruded into an appropriately sized conduit.
One problem associated with carbon filled polymeric materials relates to the propensity of the surfaces of particles of carbon black to adsorb or otherwise react with their environment. When conductive carbon fillers are dispersed in polymeric chemical transport lines, the chemicals and additive components of the polymer interact with the surface of the carbon black filler particles. Once a sufficient amount of the chemical or additive adsorbs or reacts, the surface of the carbon filler becomes non-conductive. This in turn results in a non-conductive polymeric composition which is susceptible to failure and is potentially a serious safety hazard when used to transport chemicals.
Another problem with carbon filler material involves the catalytic characteristics of carbon surfaces. In certain systems carbon surfaces may catalyze reactions between the surrounding polymer matrix and an adsorbed substance leading to mechanical weakening of the chemical transport line.
Accordingly, it is an object of the present invention to provide electrically conductive polymeric compositions useful for fabricating devices having long term electrical conductivity characteristics.
It is additionally an object of the present invention to provide an electrically conductive carbon filled polymeric composition which maintains its conductive properties and mechanical integrity after exposure to reactive and hostile environments.